CARACTERÍSTICAS DEL SERVICIO DE ASEO

Figure 4.4.1 Cyclic voltammetry curves showing first scans of (a and b) reduction and (c and d) oxidation scans of 10 mM of CuCl2, 100 mM of KCl and 100 mM of additives on (a and c) Au and (b and d) Mo substrate. Blue arrows indicate

negative shifts in (a) reduction and (c) oxidation peaks. The additives were (i) thiourea, (ii) thiosulfate, (iii) citric acid, and (iv) no additive. The solution pH was between 1.5 to 2.0.

Figure 4.4.1 shows cyclic voltammograms of 10 mM CuCl2 and 100 mM of KCl as supporting

A second reduction peak at -0.07 V is observed. In acidic aqueous media with chloride, the CuCl2-

complex is dominant,138 as the Cu2+ to Cu+ reduction is strongly catalysed by chlorides.139 The reduction of Cu2+ follows the pathway below:140

Cu

+ 2Cl

+ e

→ CuCl

Equation 4.4.1

CuCl

→ CuCl

+ Cl

Equation 4.4.2

CuCl

+ e

→ Cu + Cl

Equation 4.4.3

The broad peak at +0.12 V could be explained by the initial reduction of Cu2+ to Cu+ followed by formation of the CuCl2- complex. The CuCl2- complex adsorbs on the substrate as insoluble CuCl and

is further reduced to Cu as the potential decreases to -0.07 V.

The increase in current from -0.7 V to more negative values is attributed to the hydrogen evolution reaction. Interestingly, a peak was observed at -0.9 V for the case without additives (Figure 4.4.1a, black line). This suggests the reaction becomes mass transport limited.

In the positive sweep without any additives, two main oxidation peaks were identified at +0.13 V and +0.34 V, which represents the two single-electron processes of Cu → Cu+ → Cu2+ (Figure 4.4.1c, black line).

In the case with 100 mM citric acid, reduction and oxidation peaks appear at very similar values, signifying that citric addition at pH values of 1.5 to 2.0 does not show any significant complexing effect on Cu2+ (Figure 4.4.1a and c, red line). This agrees with the predictions from Gougaud et al. based on calculated speciation diagrams, suggesting that citric acid only shows complexing effect at pH above 2.64

With addition of 100 mM thiosulfate, a significant peak shift in deposition potential from -0.07 V to -0.56 V (Figure 4.4.1a, blue line) was observed. This negative shift of 0.49 V is attributed to complexation of Cu+ by thiosulfate. Etschmann et al. reported that among the Cu-thiosulfate complexes, the [Cu(S2O3)3]5- complex has the highest stability constant.104 They also reported that in

electrolytes with chlorides present, complexes such as [Cu(S2O3)Cl(H2O)]2- and [Cu(S2O3)Cl2]3- are

suggested to exist. An oxidation peak was observed in the positive sweep with a peak value of -0.31 V (Figure 4.4.1c, blue line). Unlike in the case with 100 mM citric acid and without additives, only one oxidation peak was observed. The presence of only one oxidation peak could be explained by the uptake of any oxidized Cu by the additive before stabilization of the surface by Cl-.

A similar trend was also observed for thiourea with a deposition potential of -0.59 V to -0.64 V (Figure 4.4.1a, magenta line) followed by an oxidation peak at -0.23 V (Figure 4.4.1c, magenta line). Farndon et al. have suggested the following relevant reactions:105

2Cu

+ 2(NH

)

CS → 2Cu

+ [-SC(=NH)NH

]

+ 2H

Equation 4.4.4

Cu

+ x(NH

)

CS → [Cu[(NH

)

CS]

]

(x = 1 to 4)

Equation 4.4.5

[Cu[(NH

)

CS]

]

+ e

→ Cu + [(NH

)

CS]

Equation 4.4.6

Thiourea reduces Cu2+ to Cu+ while it is oxidized to form formamidine disulfide ([-SC(=NH)NH2]2).

The Cu+ ion is then complexed by thiourea, and subsequently reduced to Cu at the electrode. A reproducible reduction peak was observed at around -0.1 V, and is attributed to the reduction of the CuCl2- complex. The dominant oxidation peak at -0.23 V compared to the small oxidation peak at

+0.13 V suggests that the main Cu deposition mechanism occurs via the Cu-thiourea complex instead of the CuCl2- complex. Furthermore, literature suggests that the stability constant (log β) of

Cu-thiourea complex is between 11.00 to 15.40,107 which is higher than the CuCl2- complex, which

has a log β of 5.48.141 This suggests that most of the Cu species exist in the form of the Cu-thiourea complex as suggested by the speciation diagram in Figure 4.3.4a.

An increase in oxidation current was observed with thiosulfate from +0.4 V to more positive values. However, this phenomenon was not observed in the case of Zn2+ and Sn2+ (Figure 4.5.1b and d), which implies that the oxidation is only observed in the presence of Cu2+. Zhang and Nicol have reported Cu-catalysed Au dissolution in the presence of thiosulfate by oxidation of the Au by the Cu-thiosulfate complex.142 They also reported that dissolution is more pronounced at lower pH values.

Oxidation peaks above +0.2 V were observed for thiosulfate and thiourea but not in citric acid and without additive. Scans were also compared with solutions with (Figure 4.4.2a) KCl only, (Figure 4.4.2b) KCl and thiourea, and (Figure 4.4.2c) KCl, thiourea, and Cu2+, and oxidation peaks above +0.2 V were observed when thiourea was present.

Hence the oxidation peaks above +0.2 V for Figure 4.4.1 is attributed to gold leaching at positive potentials by thiosulfate and thiourea,143, 144 evidenced by the disappearance of the gold film after the experiment. In the case of Zn2+ and Sn2+ (Figure 4.5.1c and d), oxidation peaks above +0.2 V were not observed for thiosulfate. Feng and van Deventer reported that the presence of impurities such as Zn (over 10 mg/L) decreased the dissolution of Au even with thiosulfate present.145

were identified at +0.12 V and -0.10 V for citric acid and without additive (Figure 4.4.1b, black and red lines). With addition of thiosulfate and thiourea, the reduction peak shifted to -0.46 V and -0.41 V respectively (Figure 4.4.1b, blue and magenta lines). A main reduction peak at -0.68 V was also observed without additive, with thiosulfate, and with thiourea. It was not observed in citric acid as it is masked by hydrogen evolution. Pre-treatment of the molybdenum substrate at -0.8 V did not appear to have any effect on the peak at -0.68 V. Lu and Clayton found only weak Cl 2p signals from XPS on molybdenum in 0.1 M HCl, indicating that chlorine-containing surface complexes were not the major species.146 Like Au, the oxidation peaks for Mo also showed a negative shift with the addition of thiosulfate and thiourea but not for citric acid (Figure 4.4.1d). The large oxidative currents at potentials greater than +0.2 V are attributed to transpassive dissolution of the Mo layer as soluble HMoO4- in the following equations:147

MoO

+ 2H

O → HMoO

+ 3H

+ 2e

Equation 4.4.7

MoO

+ H

O → HMoO

+ H

Equation 4.4.8

Figure 4.4.2 Positive linear sweep voltammetry on Au with (a) KCl, (b) KCl and thiourea, and (c) KCl, thiourea, and CuCl2.

Electrolyte contained 100 mM of KCl, 100 mM of thiourea, and 10 mM of CuCl2. Black dashed arrow shows direction

of scan. The solution pH was between 1.5 to 2.0.

From this section, the addition of citric acid did not show any significant changes to the redox peaks for Cu2+. However, thiosulfate and thiourea both result in a significant negative shift in both